EPS inclusion to reduce vertical stresses on shallow tunnels

Expanded polystyrene (EPS) has long been used to reduce stresses acting on buried structures. In this study, the efficiency of utilising EPS in reducing vertical stresses acting on cut-and-cover tunnels was investigated. To gauge this, short- and long-term shear strength parameters of EPS with densities of 25, 30, and 35 kg/m3 were determined. Interface friction of EPS with various materials was measured considering the use of geotextile as a protective cover for EPS. Laboratory testing included unconfined compression, creep strain based on time-temperature-stress superposition, and modified direct shear. Then, a numerical model was developed and verified to simulate shallow tunnels with EPS inclusion. A parametric study was conducted using EPS with various thicknesses to tunnel depth ratios (tg/z1). It was found that the total vertical stress acting on the tunnel can be reduced by 35% using tg/z1 limited to 0.4. Also, the effect of positive soil arching amplified at tg/z1 ≥ 0.6, leading to a reduction that exceeded 50% in the net vertical stress acting on the tunnel. The magnitude of vertical displacement was found to be directly proportional to tg/z1, but with an influence zone that can be controlled by increasing the EPS thickness. © 2019 Thomas Telford Ltd.